In the earliest prior art the method of locating the position of a remote transmitter was to utilize a direction finding (DF) triangulation technique where signals from the transmitter are received at widely spaced DF antenna sites. A line-of-bearing (LOB) measurement to the transmitter is measured at each antenna site. When the LOBs are plotted on a map they intersect at the transmitter location. The accuracy of this intersection is directly related to the accuracy of these lines-of-bearing.
A typical radio frequency interferometer system computes an AOA to a transmitter by utilizing the phase difference of the transmitter signal arriving at individual antennas of an array. A remote transmitter is located by utilizing the amplitude and phase difference of a signal from the transmitter arriving at different antennas of an antenna array. The phase measurements of the interferometer can be AOA ambiguous if the baselines of the antenna array, that is the separation of the antennas of the array and used to measure signal phase, is greater than half the wavelength of the incoming signal. The number of ambiguous AOA's is closely approximated by the ratio of the interferometer baseline to the emitter wavelength and one key to successful emitter geolocation estimation is to correctly resolve these potential ambiguities by careful DF array design and measurement processing.
As the interferometer baseline length increases, thereby increasing the number of AOA ambiguities, the phase measurement accuracy increases, and the rapidity of transmitter ranging to the required accuracy improves due to the more accurate bearing measurements. Thus, the desire for accurate bearing measurements, requiring a long interferometer baseline, conflicts with the need for robust phase or AOA ambiguity resolution, which is easier to accomplish with a short baseline. In addition, long baselines are difficult to achieve when the receiving apparatus is on an aircraft.
One technique to overcome this complexity is to use an AOA ambiguous long baseline interferometer or (LBI). One approach to passive ranging utilizing an LBI to resolve the AOA ambiguities, which are identical to the antenna array grating lobes, is by phase tracking the emitter signal during the relative motion of the platform, such as an aircraft, containing the DF antenna array. Lobe tracking is utilized in which 2n solutions are set up for an array with a length of nλ, where λ is the wavelength of the transmitter signal and n is an integer. Only one of the potential solutions converges to the true solution with the remainder being rejected as diverging. Phase tracking is continuously performed by the lobe tracking process in order to eliminate all but one of the potential solutions so that the ambiguity integer m, which determines the number of 2π cycles which must be added to the interferometer phase measurement to correctly identify the correct AOA, is determined.
The phase tracking approach has certain drawbacks. For instance, the signal can be interrupted by terrain blockage or intermittent emitter operation and the trend on the ambiguity integer m (phase tracking) is lost. Additionally, vibration can distort the trend and hinder correct ambiguity resolution. Observer attitude motion can cause large changes in the ambiguity integer, m. These changes are difficult to separate from translational motion relative to the emitter.
Another drawback to the phase tracking approach is that the number of lobes, i.e. potential solutions, is determined by the ratio of d/λ, where d is the baseline length and λ is the signal wavelength.
U.S. Pat. No. 5,835,060 entitled “Self-resolving LBI Triangulation” also teaches a long base line interferometer (LBI) system for determining the position of a transmitter. The system has two antennas and the phase differences between the signals received by the antennas at each end of the long base line are monitored as the interferometer moves along a measurement path to obtain repetitive phase difference measurements distributed along the measurement path. To determine the location of the transmitter, a cost function is evaluated to select one of a set trial grid points for the transmitter. The position of the transmitter is then estimated by least squares convergence using the selected trial grid point as a starting point.
The measured phase difference corresponds to the angle of arrival of the transmitted signal plus an unknown constant minus an unknown integer multiple of 2π. The measured phase difference (in radians) can be expressed as follows:
                    Φ        =                              [                                          Φ                0                            +                                                                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    l                                    λ                                ⁢                                                                  ⁢                Cos                ⁢                                                                  ⁢                θ                                      ]                    ⁢                                          ⁢          mod          ⁢                                          ⁢          2          ⁢                                          ⁢          π                                    (                  EQ          ⁢                                          ⁢          1                )            in which Φ is the measured phase difference, Φ0 is an unknown constant, L is the length of the baseline of the LBI antenna, λ is the wavelength of the transmitted signal and θ is the angle of arrival of the transmitted signal relative to the LBI base line. This approach has a limitation in that it cannot handle signals from arbitrarily polarized transmissions. Computing directions to arbitrarily polarized transmissions, requires both phase and amplitude RF measurements on more than two antennas.
Another prior art passive ranging approach utilizes a short baseline/long baseline interferometer or SBI/LBI system in which at least two SBI measurements separated in time are needed to resolve the two-antenna element LBI ambiguity. This approach is described in U.S. Pat. No. 4,734,702 “Passive Ranging Method and Apparatus”.
U.S. Pat. No. 5,039,991, entitled “Perturbation modeling system for use in processing direction-finding antenna outputs” teaches and claims a system for compensating for perturbations of received electromagnetic radiation caused by the various surfaces of an aircraft by correlating antenna outputs to representative data in a database that is empirically derived during calibration of the system at a plurality of scaled electromagnetic radiation frequencies to determine the proper azimuth and elevation associated with transmissions.
See also a paper by N. Saucier and K. Struckman, Direction Finding Using Correlation Techniques, IEEE Antenna Propagation Society International Symposium, pp. 260-263, June 1975, which teaches the same concepts as taught in U.S. Pat. No. 5,039,991.
U.S. Pat. No. 4,734,702 discloses two approaches utilizing SBI/LBI. One approach locates the target with SBI derived measurements and uses the SBI range to predict the LBI phase change. This approach requires some SBI location convergence before improving it with the LBI. The use of the SBI phase difference to initially compute a slant range means this method will not initially converge faster than a more conventional SBI-only system until range accuracy sufficient to resolve the LBI has been achieved. In order to overcome this slow initial convergence to the range estimate, a second technique is used instead, if the slow initial convergence is intolerable.
The second technique utilizes the SBI unit direction-of-arrival vector (DOA) to predict the LBI phase change. This technique does not require location to any accuracy before differentially resolving the LBI with sequential SBI measurements, and hence provides rapid convergence to an accurate range estimate. However, this SBI/LBI technique has the drawback of limiting the SBI/LBI baseline ratio, and requiring the use of a medium baseline interferometer (MBI) in many cases. It also requires a two dimensional (2-D) SBI to measure emitter direction of arrival or DOA, as opposed to just a one dimensional interferometer array measuring AOA.
The baseline restriction existing in the conventional SBI/LBI approach necessitates the addition of more antenna elements and receivers to obtain the LBI baseline required to achieve the desired range accuracy quickly. This introduces extra complexity, cost and weight to a system.
One technique to overcome the extra complexity, cost and weight is to use an AOA ambiguous long baseline interferometer or LBI. One early approach to passive ranging utilizing an LBI which may have only two antenna elements is disclosed in U.S. Pat. No. 3,935,574 by Pentheroudaki. This approach resolves the AOA ambiguities, which are identical to the antenna array grating lobes, by phase tracking the emitter signal during the relative motion of the platform containing the antenna array. Such lobe tracking is utilized in which (2n) solutions are set up for an array with a length of nλ, where λ is the transmitter signal wavelength and n is an integer. Only one of the potential solutions converges to the true solution with the remaining possible solutions being rejected as diverging. Phase tracking is continuously performed by a lobe tracking process in order to eliminate all but one of the potential solutions so that the ambiguity integer m, which determines the number of 2π cycles which must be added to the interferometer phase measurement to correctly identify the correct AOA, is determined.
As described above, a typical DF interferometer system locates a remote transmitter by utilizing the phase difference of the transmitter signal arriving at the individual antennas. DF accuracy of such systems is directly related to DF array aperture size which is determined by the spacing between multiple antennas of antenna array of the DF system. All other things being equal, larger DF apertures increase LOB accuracy generating more accurate transmitter fixes. However, simply increasing DF aperture sizes without increasing the number of DF antennas leads to large amplitude correlation side lobes and a real potential for large errors. Therefore, such prior art DF systems require many antennas and DF receivers and are very expensive. The need for more antennas and more DF receivers negatively affects their use on aircraft.
In summary, accurate transmitter geo-location computations require long baseline interferometer (LBI) accuracy's. Theoretically only three antenna elements are required for unique 2π radians azimuth coverage, but conventional DF interferometer systems must utilize a number of additional antenna elements and receivers to control the gross error rate of a resulting from the inherent AOA ambiguity in phase measurement.
As will be appreciated, the number of antenna elements required by airborne DF interferometer systems leaves a limited amount of space for other sensors on the aircraft's exterior. Thus, it is desirable to provide a DF interferometer system for aircraft that needs only a few antenna elements while providing the same or greater transmitter location accuracy as prior art systems.